The story caused a sensation, as media discussed what could make rocks catch on fire. By Friday, California environmental health officials had an answer, or at least part of one: two of the rocks were covered in phosphorus, an element that’s known for igniting into a fierce white flame when it’s exposed to air. Near as they can tell, as long as the rocks were wet with seawater, the phosphorus didn’t ignite, but after they’d dried out in the woman’s pockets over the course of the day, the phosphorus reacted explosively.

But how did the rocks get covered with phosphorus? Though the substance is mined and used in fertilizers, it isn’t very common in in the natural world in its explosive form, called white phosphorus. White phosphorous does, however, have a long history of production by militaries, who use it in flares. Unexploded military flares, presumably dropped by aircraft, have been known to wash up on beaches: Just last year flares washed up on a beach a half-hour’s drive from San Onofre. NBC reported that those flares were from military exercises going on off the coast.